Cellular foam glass is used for heat insulation, with properties of light weight, low thermal conductivity, non-moisture absorbing, and resistance to chemical corrosive materials. Cellular foam glass is frequently used in harsh conditions with moist and corrosive environments such as is present in flue gas stacks. A typical such environment exposes cellular foam glass to flue gas and undergoes wet and dry cycling, acid and other chemical exposure, and temperature variances up to 500° F. and higher. Blocks in the flue gas environment are exposed to water and the pH variation to which they are exposed can range between 1 to 9 depending on where the blocks are located in the flue gas stream. Cellular foam glass used in these environments must be resistant to such harsh exposure. Foam glass having a closed cell structure is required for a wet and dry cycling environment so that moisture stays out of the interior of the foam glass block. Otherwise, moisture that penetrates into the foam glass will expand and contract under a considerably higher coefficient of expansion than that for the cell walls inside the block, and can cause the foam glass cell walls to crack, thus damaging the foam glass block.
The harsh conditions placed upon cellular glass blocks are especially pronounced in flue gas stacks having a substantial height, often up to 300 feet in length. Blocks are normally applied in continuous contact with each other along the length of the flue gas stack. As blocks in the flue gas stack heat up, the expansion of the blocks along the length of the flue gas stack require that their coefficient of expansion be low enough to prevent crushing the blocks or to cause other failure. As such, blocks used in the flue gas environment, especially in flue gas stacks, should have properties comprising a closed cell foam having high water insolubility, excellent acid resistance, and a low coefficient of expansion, and provide sufficient strength to withstand the abrasion of particles from the high velocity flue gas exposure.
There exist known methods for producing foamed glass (cellular glass) using controlled raw materials to insure that the chemical composition of the charged mixture is pure and very consistent. The constituent materials are mixed to insure completely uniform contact of all basic chemicals with each other. In some cases, an aqueous solution of the mixed materials is blended with silica to make a slurry. The slurry is then spray dried to obtain a completely uniform mixture for foaming. Alternatively, the ingredients are melted to form a homogeneous liquid glass mixture prior to foaming. In many cases, a two-step foaming process is utilized wherein the product is foamed and then re-ground with a new charge prior to making the final foamed glass. All of the present processes, although differing in minor points from one to another, are tedious and increase the cost of the final product. The processes all start with a very consistent mixture of glass chemicals that are pure and uniform. The double firing process is costly and time consuming while using double the amount of energy to produce the product.
Other prior art processes are designed to produce a pure silica foam, and add excess boron, up to 50% by weight, to achieve lower softening temperatures for ease of foaming and then leach out the boron with hot water to produce a nearly pure silica foam. This process produces an open cell foam which is friable and weak. Blocks produced by this process, while useful for insulation against high temperatures when used in a less harsh environment, are therefore unsuitable in the harsh environment of high velocity flue gas streams undergoing cycling temperatures and wet and dry exposures. The harsh environment of the flue gas stack requires a closed-cell block. Other processes contemplate a boron content but provide no detail by which to control the boron content or achieve an appropriate coefficient of expansion suitable for the flue gas environment. As with the other processes, a higher content of boron may lead to phase separation whereby the product is water soluble and a closed cell structure is not created. Additionally, without controlling the boron content to within proper ranges, the coefficient of expansion of the produced block is subject to substantial variation. And so, foam glass blocks produced under these processes are not well-suited for the wet and dry cycling and wide range temperature environment in flue gas applications.